The "dual probe method" for measuring surface potentials of biological membranes will be expanded to treat intra-membrane ("dipole") potentials. Factors promoting or inhibiting the permeability of mitochondrial and other membranes to charged amphiphilic spin probes will be characterized. The dispersion of activities of proton pumping vesicles will be estimated with spin labeled weak acids and amines and related to maximum ("threshold") potentials. The biological significance of free radical reactions causing nitroxide destruction in illuminated inner mitochondrial membranes will be further characterized.